Catalysts for the removal of sulfur compounds from industrial gases, a process for their production and their use

ABSTRACT

Catalysts useful for the removal of volatile sulfur compounds from industrial gases composed of an inorganic, abrasion-resistant, incombustible support which is uniformly impregnated with either (a) an oxide or hydroxide of niobium or tantalum or (b) an oxide or hydroxide of manganese and an oxide or hydroxide of hafnium, lanthanum or an element in the Lanthanide Series of Elements.

BACKGROUND OF THE INVENTION

This invention relates to catalysts with which volatile sulfur compoundscan be removed substantially completely from industrial gases, to aprocess for the production of these catalysts and to their use for theremoval of sulfur compounds from industrial gases.

Sulfur-containing compounds such as, for example, carbon oxysulfide,carbon disulfide and hydrogen sulfide are common impurities ofindustrial gases such as, for example, carbon monoxide, carbon dioxide,nitrogen, methane and natural gas. They are formed, for example, in theprocessing of sulfur-containing gaseous, liquid or solid startingmaterials, such as natural gas, petroleum, coke and coal.

The removal of such sulfur-containing constituents from industrial gasesis often necessary, for example in view of catalyst damage, corrosion,unwanted secondary reactions in the organic and inorganic synthesis andalso in view of the high toxicity of the sulfur-containing components,even in low concentrations.

Whereas hydrogen sulfide as the most common impurity of carbonoxysulfide and carbon disulfide can be removed from gases by a number ofknown processes, the simultaneous removal of carbon oxysulfide andcarbon disulfide from gases is problematical.

Adsorbents, such as active charcoal, zeolites or silica gel, aregenerally known to have only a very low adsorption capacity for carbonoxysulfide and carbon disulfide. The purification of gases, for exampleby washing with liquid basic media, involves considerable effort becausethe solubility of carbon oxysulfide and carbon disulfide in such mediais minimal. In addition, working up of the spent solutions involvesenvironmental problems.

According to JP 247 084, heavy metals of the first, second and eighthsecondary groups of the periodic system and lanthanide elements areadded to lignite as a support material. The catalyst obtained in thisway is capable of adsorbing sulfur-containing compounds from industrialgases and desorbing them again at a later stage. In this case, however,the sulfur-containing components do not react chemically to form sulfurcompounds readily removable from a gas mixture, so that desorption ofthe sulfur-containing components leaves a gas mixture enriched withtoxic sulfur compounds which is ecologically undesirable.

Processes in which carbon oxysulfide and carbon disulfide are convertedinto hydrogen sulfide on solid media, generally zeolites, aluminiumoxide and the like, and the hydrogen sulfide formed is subsequentlyremoved from the gas mixture by known methods have been adopted forpractical application.

The disadvantage of such processes lies in their high energy consumptionbecause the conversion of carbon oxysulfide or carbon disulfide onlytakes place at high temperatures, generally of the order of 400° to 800°C.

A process for the removal of carbon oxysulfide from gases which is knownfrom DE-A-1 667 590 uses certain heavy metal oxides as catalysts onactive charcoal as support. This process has many disadvantages onaccount of the abrasion of the charcoal and its combustibility.

In addition, it is known from DE-A 2 203 494 that aluminium oxidecontaining molybdenum, iron, tungsten, nickel or cobalt or certaincombinations of these metals can be used as a catalyst for the reactionof carbon oxysulfide with steam. These metals are generally used in theform of sulfides. However, uneconomically high temperatures of 250° to400° C. are required for the hydrolytic reaction of carbon oxysulfideand carbon disulfide.

SUMMARY OF THE INVENTION

Accordingly, the problem addressed by the present invention was toprovide catalysts which would enable sulfur compounds to be removedsubstantially completely from gases without any of the disadvantagesmentioned above.

Surprisingly, this problem has been solved by the catalysts according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to catalysts for the removal of volatilesulfur compounds from industrial gases, the catalysts consisting of aninorganic, abrasion-resistant, incombustible support which is uniformlyimpregnated with oxides/hydroxides of the elements from the niobium andtantalum group and optionally with oxides/hydroxides of elements of thesecond main group of the periodic system or with oxides/hydroxides ofmanganese and with at least one oxide/hydroxide of elements from thehafnium, lanthanum and lanthanides group and optionally withoxides/hydroxides of elements from the tungsten and molybdenum group.

Preferred catalysts are those of which the inorganic support consists ofaluminium oxide, magnesium oxide, a spinel, more particularlymagnesium-aluminium oxide, a zeolite or mica and has a surface of 10 to500 m² /g.

In one preferred embodiment, the inorganic support contains theoxides/hydroxides of the elements in a quantity of 0.05 to 6% by weightand preferably 0.1 to 3% by weight, based on the quantity of the supportmaterial.

The inorganic support is preferably uniformly impregnated withoxides/hydroxides of manganese and with at least one oxide/hydroxide ofthe elements from the group comprising La, Ce, Pr, Nd, En, Gd, Tb, Dy,Yb and Lu and optionally with oxides/hydroxides of the elements from thetungsten and molybdenum group.

In one particularly preferred embodiment, the inorganic support isuniformly impregnated with oxides/hydroxides of manganese and with atleast one oxide/hydroxide of the elements from the La, Ce, Pr and Ndgroup and optionally with oxides/hydroxides of the elements from thetungsten and molybdenum group.

The present invention also relates to a process for the production ofthe catalysts according to the invention which is characterized in thatthe inorganic support is initially introduced in granular form with anaverage diameter of 0.1 to 20 mm and preferably 1 to 10 mm, uniformlytreated with aqueous solutions of compounds of the elements, dried andheated at 200° to 500° C. and preferably at 280° to 450° C.

Aqueous solutions of nitrates and/or acetates of the elements andammonium salts and/or alkali metal salts of the molybdates andtungstates are preferably used.

In one preferred variant of the process, the support is successivelytreated with several solutions containing one or more compounds andoptionally dried after each treatment.

The aqueous solutions of the compounds are preferably uniformly appliedby immersion of the support in the corresponding solutions or byspraying of the support with the corresponding aqueous solutions.

The catalysts according to the invention are used for the substantiallycomplete removal of sulfur compounds, more particularly COS, CS₂ andorganic sulfur compounds, more particularly mercaptans, from industrialgases, more particularly methane, CO₂, N₂, CO and natural gas.

Where the catalysts according to the invention are used for the removalof sulfur compounds from industrial gases, the industrial gases to bepurified are preferably

a) charged with steam and then

b) passed over a catalyst according to the invention at temperatures of20° to 185° C and preferably at temperatures of 20° to 80° C. and

c) are then freed from the hydrogen sulfide formed in b) and from thehydrogen sulfide present in the gas from the outset.

The hydrogen sulfide is preferably removed in c) by passing the gases b)together with air and/or oxygen over a catalyst according to theinvention at temperatures of 15° to 180° C. and preferably attemperatures of 20° to 80° C. and regenerating the catalyst, preferablywith steam, to remove the sulfur formed in c) from the catalyst.

The purification of the industrial gases preferably takes place underlow pressures, more particularly in the range from 0.9 to 3 bar.

The catalysts provide for the quantitative removal of thesulfur-containing compounds mentioned above from gases at lowtemperatures and over very short residence times, so that economy isimproved by longer useful lives and higher safety standards.

In the process according to the invention for the purification of gases,for example, a catalytic reaction of gases to be purified, for examplecontaining carbon oxysulfide or carbon disulfide, is carried out in areactor known as a hydrolysis tower, which consists of a heatablereaction tower with a bed of the catalyst according to the invention,the sulfur compounds being converted into hydrogen sulfide. The hydrogensulfide thus formed and any hydrogen sulfide already present in the gasis then oxidized in the presence of air to elemental sulfur, for examplein another reactor consisting of a reaction tower known as an oxidationtower with a bed of the catalyst according to the invention.

The element sulfur accumulating during the oxidation in the oxidationtower may readily be removed from the oxidation tower with hot steam.The catalyst bed is not affected by this treatment.

To produce the catalyst, the commercial support may be used in powderform or in particulate form. For a fixed-bed process, the catalyst isused in particulate form, for example in the form of extrudates, pills,pellets or beads with dimensions of, preferably, 0.1 to 20 mm.

In the production of the catalyst according to the invention, forexample, the inorganic support, preferably γ-Al₂ O₃, is charged withcompounds of the corresponding elements. The support thus charged isthen dried and heated to 200° to 500° C. and preferably to 280° to 450°C. The elements mentioned may be applied to the inorganic support, forexample, by impregnation or spraying with suitable salt solutions ofthese elements, followed by a drying phase and by the complete heatingphase. Drying is carried out in known manner at 80° to 130° C.,optionally in a vacuum drying unit. In the subsequent heating phase, thecompounds applied are converted into oxides/hydroxides which adherefirmly to the inorganic support. However, the elements mentioned mayalso be applied by co-precipitation of a hydroxide mixture from salts ofthe above-mentioned elements onto the support using basic compounds suchas, for example, alkali metal hydroxide or ammonia, optionally followedby washing of the soluble components with water. Suitable salts of theelements mentioned are, in particular, the acetates, chlorides, nitratesand sulfates. The subsequent drying and heating in the temperatureranges mentioned takes place over a period of 1 to 100 hours andpreferably over a period of 2 to 20 hours or over a period of 1 to 12hours and more particularly over a period of 2 to 6 hours, during whichthe temperature may even be increased within the ranges mentioned above.

The inorganic support may also be impregnated with an aqueous solutionin which all the elements are dissolved in the form of their salts.However, the elements may also be successively applied to the supporteither individually or in certain combinations by successively using thecorresponding solutions. After each impregnation, the catalyst may bedried so that it has sufficient absorbency for the followingimpregnation.

A particularly effective catalyst for the purification of gases inaccordance with the invention is obtained using γ-Al₂ O₃ granules whichare impregnated with a solution of a niobium compound in such a quantitythat the catalyst formed has a niobium content of 0.2 to 6% by weightand preferably 0.5 to 4% by weight. To produce the catalyst, niobic acid(Nb₂ O₅ ·nH₂ O) and Al₂ O₃ may be mixed in powder form and the resultingmixture converted into pellets. Niobic acid and aluminium oxide may beused in a quantitative ratio of 5:95 to 95:5 and preferably in aquantitative ratio of 10:90 to 80:20. Instead of niobium compounds, thecorresponding tantalum compounds or mixtures of niobium and tantalum mayalso be used. In a preferred embodiment, the niobic acid used containstantalic acid emanating from its natural origin in a quantity of 0,001to 10 mol-%, based on the total mols of niobic and tantalic acid.

An addition of Ca, Mg, Sr and/or Ba in the form of their oxides orhydroxides is suitable for the after-treatment of the niobium/tantalumcatalysts. To this end, the catalyst is impregnated with a correspondingsolution of alkaline earth metal compounds. The nitrates of the alkalineearth metals are advantageously used for this purpose because theydecompose on heating to relatively high temperatures. The alkaline earthmetals are applied to the catalyst in such a quantity that the totalalkaline earth metal content is preferably 0.5 to 5% by weight. Thepercentage content of the individual alkaline earth metals may be variedas required.

Another particularly effective catalyst for the purification of gases inaccordance with the invention is obtained by applying compounds ofcerium and manganese to γ-Al₂ O₃ in the form of extrudates and heatingthe support thus charged after drying to temperatures of 200° to 450° C.The support thus charged is then additionally impregnated or sprayedwith a solution of a tungsten salt, followed by another drying phase andthen by heating at 200° to 450° C.

The following Examples are intended to illustrate the invention withoutlimiting it in any way.

Production of the Catalyst Example 1

1,000 g of a commercial γ-Al₂ O₃ with a specific surface of 350 m² /gand a mean particle diameter of 2 to 5 mm are impregnated with asolution consisting of 62 g of Ce(NO₃)₃ ·6H₂ O, 91.5 g of Mn(NO₃)₂ ·4H₂O and 375 g of water. After intermediate drying at 90° to 100° C., thespherical catalyst particles are treated with an equivalent quantity ofsodium hydroxide to precipitate the cerium and manganese compounds ontothe aluminium oxide. The spherical catalyst particles were then washedfree from nitrate and redried. The catalyst was then heated for 3 h at400° C.

200 g of the catalyst thus produced was additionally impregnated with asolution which had been prepared from 4.2 g of Na₂ WO₄.2H₂ O and 75 g ofwater. The catalyst was then dried for 18 h at 120° C. in a water jetvacuum and subsequently heated for 3 h at 400° C. The catalyst thusproduced is used for converting the sulfur compounds present inindustrial gases (for conditions, see Table 1 and test procedure).

Example 2

400 g of a commercial γ-Al₂ O₃ with a specific surface of 350 m² /g anda particle diameter of 2 to 6 mm were impregnated with a solution whichhad been prepared from 25 g of La(NO₃)·6H₂ O, 35.7 g of Mn(CH₃ COO)₂.4H₂O and 80 g of distilled water. The aluminium oxide thus impregnated wasthen dried for 18 hours at 100° C. in a water jet vacuum andsubsequently heated for 4 hours at 400° C.

The catalyst thus produced is used for the purification of industrialgases (for conditions, see Table 2 and test procedure).

Example 3

400 g of the same Al₂ O₃ granules as in Example 1 are impregnated with aniobium pentachloride solution which had been prepared as follows: 11.64g of NbCl₅ and 11.64 g of NaCl were first dissolved with 30 g of wateron a steam bath and the resulting solution was subsequently diluted withanother 70 g of water. After intermediate drying at 100° C. in a waterjet vacuum, the NbCl₅ -impregnated aluminium oxide was impregnated for 1hour with 418 g of an aqueous 2.2% NH₃ solution and washed with wateruntil free from chloride. After further intermediate drying, 200 g ofthe aluminium oxide thus pretreated were impregnated with a solutionwhich had been prepared from 11.8 g of Ca(NO₃)₂ ·4H₂ O, 21.1 g ofMg(NO3)₂ ·6H₂ O and 30 g of water. The aluminium oxide was then driedfor 18 hours at 100° C. in a water jet vacuum and subsequently heatedfor 4 hours at 400° C.

Example 4

40 g of niobic acid powder, 160 g of γ-Al₂ O₃ powder, specific surface350 m² /g, and 7 g of graphite powder were intensively mixed andconverted into 5 mm diameter pellets.

Example 5

1,000 g of a commercial γ-Al₂ O₃ with a specific surface of 350 m² /gand a particle diameter of 2 to 5 mm were impregnated with a solutionconsisting of 62 g of Ce(NO₃)₃ ·6H₂ O, 91.5 g of Mn(NO₃)₂ ·4H₂ O and 375g of water. After intermediate drying at 90° to 100° C., the catalystbeads were treated with an equivalent quantity of sodium hydroxide toprecipitate the cerium and manganese on the aluminium oxide. Thespherical catalyst particles were then washed free from nitrate andredried. The catalyst was then heated for 3 hours at 400° C.

Purification of the Gases (Test Procedure)

The gas to be purified is first passed through a water-filled tank andsaturated with steam. The exact gas composition (1) of the gas to beconverted is determined by means of a gas chromatograph. The gas is thenintroduced into a hydrolysis tower (500 ml). The hydrolysis towerconsists of a double-walled vessel, the temperature of the reactiontower being controllable by means of a liquid, and is filled withcatalyst according to the invention. The converted gas leaves thehydrolysis tower. Its composition is then measured (2). The gas to bepurified then enters an oxidation tower (500 ml). The oxidation towerhas the same construction as the described hydrolysis tower and isfilled with the same catalyst according to the invention. At the sametime, air is introduced into the oxidation tower in an at leaststoichiometric quantity, based on oxygen and hydrogen sulfide.

The gas now freed from the sulfur-containing compounds leaves theoxidation tower and is tested for purity (3) (see Tables 1 to 5).

In general, 200 to 400 ml of catalyst were used.

                  TABLE 1                                                         ______________________________________                                        Catalyst according to Example 1                                               Industrial gas: N.sub.2                                                                      Concentration in the gas                                       Gas    Temperature during                                                                          of      of    of                                         through-                                                                             conver-           COS   COS   COS   of H.sub.2 S                       put    sion     oxidation                                                                              at (1)                                                                              at (2)                                                                              at (3)                                                                              at (3)                             [1/h]  [°C.]                                                                           [°C.]                                                                           [vpm] [vpm] [vpm] [vpm]                              ______________________________________                                        300    35       35       9068  3244  107   ≦10                         300    40       40       9068  2166  ≦10                                                                          ≦10                         300    48       48       9068   964  ≦10                                                                          ≦10                         300    60       69       8988   ≦10                                                                         ≦10                                                                          ≦10                         ______________________________________                                         vpm  volume parts per million                                                 at (1)  concentration of COS before conversion                                at (2)  concentration of COS after conversion                                 at (3)  concentration of COS and H.sub.2 S after oxidation               

                  TABLE 2                                                         ______________________________________                                        Catalyst according to Example 2                                               Industrial gas: N.sub.2                                                                      Concentration in the gas                                       Gas    Temperature during                                                                          of      of    of                                         through-                                                                             conver-           COS   COS   COS   of H.sub.2 S                       put    sion     Oxidation                                                                              at (1)                                                                              at (2)                                                                              at (3)                                                                              at (3)                             [1/h]  [°C.]                                                                           [°C.]                                                                           [vpm] [vpm] [vpm] [vpm]                              ______________________________________                                        300    32       55       9704  6360  301    74                                300    57       57       9705  732   ≦10                                                                          ≦10                         300    63       63       9705  ≦10                                                                          ≦10                                                                          ≦10                         ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Catalyst according to Example 3                                               Industrial gas: N.sub.2                                                                      Concentration in the gas                                       Gas    Temperature during                                                                          of      of    of                                         through-                                                                             conver-           COS   COS   COS   of H.sub.2 S                       put    sion     Oxidation                                                                              at (1)                                                                              at (2)                                                                              at (3)                                                                              at (3)                             [1/h]  [°C.]                                                                           [°C.]                                                                           [vpm] [vpm] [vpm] [vpm]                              ______________________________________                                        300    45       45       10263 3552   54   ≦10                         300    60       60       10458 439   ≦10                                                                          ≦10                         300    65       65       10193 ≦10                                                                          ≦10                                                                          ≦10                         ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Catalyst according to Example 4                                               Industrial gas: N.sub.2                                                                      Concentration in the gas                                       Gas    Temperature during                                                                          of      of    of                                         through-                                                                             conver-           COS   COS   COS   of H.sub.2 S                       put    sion     Oxidation                                                                              at (1)                                                                              at (2)                                                                              at (3)                                                                              at (3)                             [1/h]  [°C.]                                                                           [°C.]                                                                           [vpm] [vpm] [vpm] [vpm]                              ______________________________________                                        300    50       50       8927  2124  102   ≦10                         300    65       65       9039  589   ≦10                                                                          ≦10                         300    70       70       9039  ≦10                                                                          ≦10                                                                          ≦10                         ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Catalyst according to Example 5                                               Industrial gas: N.sub.2                                                                      Concentration in the gas                                       Gas    Temperature during                                                                          of      of    of                                         through-                                                                             conver-           COS   COS   COS   of H.sub.2 S                       put    sion     oxidation                                                                              at (1)                                                                              at (2)                                                                              at (3)                                                                              at (3)                             [1/h]  [°C.]                                                                           [°C.]                                                                           [vpm] [vpm] [vpm] [vpm]                              ______________________________________                                        300    70       70       9835  1098  103   ≦10                         300    90       90       9835   220  ≦10                                                                          ≦10                         300    110      110      9635   ≦10                                                                         ≦10                                                                          ≦10                         ______________________________________                                    

As the test results show, the sulfur compound COS, for example, isconverted substantially completely into H₂ S by the catalyst accordingto the invention so that, as H₂ S, it can then be oxidized substantiallycompletely to sulfur and hence removed from the gas. The sulfur adheresto the material (for example to the catalyst according to the invention)in the oxidation tower where it can be simply driven out with steamwithout damaging the material.

We claim:
 1. A catalyst for the removal of volatile sulfur compoundsfrom industrial gases comprising an inorganic, abrasion-resistant,incombustible support that is uniformly impregnated with(a) an oxide orhydroxide of(1) manganese and (2) hafnium and (b) an oxide or hydroxideof(3) tungsten or (4) molybdenum.
 2. The catalyst of claim 1 in whichthe inorganic support is selected from the group consisting of aluminumoxide, magnesium oxide, spinels, zeolites and mica.
 3. The catalyst ofclaim 2 in which the inorganic support has a surface area of from 10 to500 m² /g.
 4. The catalyst of claim 1 in which the inorganic support isa magnesium-aluminum oxide.
 5. The catalyst of claim 4 in which theinorganic support has a surface area of from 10 to 500 m² /g.
 6. Thecatalyst of claim 1 in which the total amount of oxide and/or hydroxide(a) present is from about 0.05 to about 6% by weight based on the weightof the inorganic support material.
 7. The catalyst of claim 1 in whichthe total amount of oxide and/or hydroxide (a) present is from about 0.1to about 3% by weight, based on the weight of the inorganic supportmaterial.
 8. A process for the production of the catalyst of claim 1comprising(a) treating the inorganic support which is in granular formwith an aqueous solution of(1) a compound of(i) manganese and (ii)hafnium which is capable of forming an oxide and/or hydroxide and (2) acompound of tungsten or molybdenum which is capable of forming an oxideor hydroxidein a manner such that the support will be uniformlyimpregnated with the aqueous solution, b) drying the impregnated supportand c) heating the impregnated support to a temperature of from about200° to about 500° C.
 9. The process of claim 8 in which the granularsupport has an average diameter of from about 0.1 to about 20 mm. 10.The process of claim 8 in which the granular support has an averagediameter of from about 1 to about 10 mm.
 11. The process of claim 8 inwhich step c) is carried out at a temperature of from about 280 to about450° C.
 12. A process for the production of a catalyst for the removalof volatile sulfur compounds from industrial gases comprisinga) treatingan inorganic, abrasion-resistant, incombustible support which isgranular in form with an aqueous solution of(1) a compound of niobium ortantalum which is capable of forming niobium oxide or niobium hydroxideor tantalum oxide or tantalum hydroxide, or (2) a compound of(i)manganese and (ii) hafnium, lanthanum or an element of the LanthanideSeries of the Periodic System of Elements which is capable of forming anoxide and/or hydroxide, and (3) at least one nitrate, sulfate, chloride,oxide, hydroxide, acetate, ammonium salt or alkali metal salt of amolybdate or tungstate.
 13. The process of claim 8 in which the supportis successively treated with two or more solutions containing at leastone required oxide and/or hydroxide.
 14. The process of claim 13 inwhich the support is dried after each of the successive treatments. 15.The process of claim 8 in which step a) is carried out by immersing thesupport in the aqueous solution.
 16. The process of claim 8 in whichstep a) is carried out by spraying the support with the aqueoussolution.
 17. A process for removing sulfur compounds from an industrialgas comprising contacting a sulfur compound-containing industrial gaswith a catalyst comprising an inorganic, abrasion-resistant,incombustible support that is uniformly impregnated with(a) niobiumoxide, niobium hydroxide, tantalum oxide or tantalum hydroxide or (b) anoxide or hydroxide of(i) manganese and (ii) hafnium, lanthanum or anelement from the Lanthanide Series of the Periodic System of Elements.18. A process for removing COS, CS₂ and organic sulfur compounds frommethane, carbon dioxide, nitrogen, carbon monoxide or natural gascomprising contacting the methane, carbon dioxide, .nitrogen, carbonmonoxide or natural gas with a catalyst comprising an inorganic,abrasion-resistant, incombustible support that is uniformly impregnatedwith(a) niobium oxide, niobium hydroxide, tantalum oxide or tantalumhydroxide or (b) an oxide or hydroxide of(i) manganese and (ii) hafnium,lanthanum or an element from the Lanthanide Series of the PeriodicSystem of Elements.
 19. A process for removing sulfur compounds from anindustrial gas comprisinga) charging the industrial gas with steam, b)passing the gas from a) over a catalyst comprising an inorganic,abrasion-resistant, incombustible support that is uniformly impregnatedwith(1) niobium hydroxide, niobium oxide, tantalum oxide or tantalumhydroxide or (2) an oxide or hydroxide of(i) manganese and (ii) hafnium,lanthanum or an element from the Lanthanide Series of the PeriodicSystem of Elements at a temperature of from about 20° to about 185° C.,and c) removing hydrogen sulfide.
 20. The process of claim 19 in whichstep b) is carried out at a temperature of from about 20° to about 800°C.
 21. The process of claim 19 in which step c) is carried out by mixingthe gas from b) with air and/or oxygen and passing this mixture over thecatalyst at a temperature of from about 15° to about 180° C.
 22. Theprocess of claim 19 in which step c) is carried out by mixing the gasfrom b) with air and/or oxygen and passing this mixture over thecatalyst at a temperature of from about 20° to about 80° C.
 23. Theprocess of claim 19 in which the catalyst used in b) is regenerated withsteam.
 24. The process of claim 19 in which each of steps a), b) and c)is carried out at a pressure of from about 0.9 to about 3 bar.